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Kusatsu-Shirane volcano is a complex of overlapping cones and 3 lake-filled craters (Karagama, Yugama, Mizugama) at the summit. It is located 150 km NW of Tokyo.
All historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. There are fumaroles and hot springs around the flanks of the volcano, and many rivers draining from the volcano are acid. Similar to Ijen volcano, the crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

Kusatsu-Shirane volcano news & activity updates:

Kusatsu-Shirane volcano (Japan): short, but violent explosive eruption with no warning, one fatality

Wednesday Jan 24, 2018 09:19 AM | AUTEUR : T

Eruption of Kusatsu-Shirane yesterday (JNN)

An unexpected, relatively intense but short-lived explosive eruption occurred yesterday at the volcano, which is home to a popular ski resort. Tragically, the eruption triggered an avalanche that killed one person and ballistic projectiles wounded several others who were near the vent at the time of the explosion.

With apparently no precursors, the eruption started shortly before 10 am from a vent in the valley between the young cinder cones of Aino Mine and the twin cone of Motoshirane San, both in the saddle south of the main summit cone. One of the ski lifts and runways pass very close to the vent.
Numerous skiers were only few hundreds meters away when the activity started to eject strong, vertical jets of steam mixed with ash and debris that rose a few 100 meters and sent blocks to up to 250 m distance.

The video taken by the webcam shows the activity with blocks falling on the snow, on trees and hitting the cable car:

The explosive activity decreased quickly again and might very well remain a single isolated event, similar to almost all known historic activity from the volcano. The Japanese Meteorological Survey raised the alert level from 1 to 3 (on a scale of 1-5).

It seems that the eruption was entirely phreatic or, more likely hydrothermal in nature, i.e. driven by steam explosions, and did not involve any fresh magma reaching the surface. There is no indication that magma was involved in the process; the observed activity is likely to be the result of overheated ground water that flashed into steam as a rapid adjustment of accumulated instability in the shallow hydrothermal system of the volcano.
The complex of the Kusatsu-Shirane volcano is known for its intense hydrothermal activity, which indicates that its shallow edifice is rich in circulating hot fluids; an acid lake occupied the summit crater of Shirane-San and there are many fumarole fields on the volcano; some of them were formerly were exploited to extract sulfur in up to 5 mines (now closed); numerous hot springs and spas are also found on the flanks of the volcano.

As is often the case with phreatic or hydrothermal explosions, there were probably no precursors; this of course reminds the tragic disaster of the nearby Ontake volcano eruption in 2014, which also was entirely steam-driven, had no significant or usable precursor signals, and killed more than 50.
A bit surprising was the site of the eruption, which is outside any of the younger craters of the volcano, and in an area where there had previously not been any particular activity such as fumaroles or fissures.

Actualités précédentes

JMA reported that during 8-15 August volcanic earthquakes continued at Kusatsu-Shiranesan’s crater, although they had decreased from early August and tremor was absent. The Alert Level remains at 2 (on a scale of 1-5). ... [details]